The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling line pressure in a hydraulic circuit for the transmission.
A known control system for a continuously variable belt-drive transmission disclosed in U.S. Pat. No. 4,369,675 comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependence on driving conditions. The system is provided with a hydraulic circuit including a pump, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to decide the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The line pressure is controlled to prevent the belt from slipping on pulleys in order to transmit the output of the engine.
Japanese Patent Laid Open No. 57-161347 discloses a system for controlling the line pressure in proportion to the output torque of an engine. In the system, when a throttle valve of an engine is closed, the line pressure is kept at a sufficient value to hold the belt on pulleys so as to provide engine-braking effect at deceleration of a vehicle. Generally, the clutch is automatically disengaged when the speed of the vehicle decreases below a predetermined value in order to prevent the stall of the engine. Even if the clutch is disengaged, the line pressure is kept at a proper value in order to ensure the rapid response of the transmission to re-acceleration of the vehicle in accordance with depression of an accelerator pedal. After the disengagement of the clutch, braking force of Tf.times.n (where Tf is friction torque between the belt and pulleys, and n is total reduction ratio of the transmission) exerts on the vehicle. The friction torque Tf is proportional to the line pressure. Accordingly, comparatively large braking force exerts on the vehicle body, dependent on the line pressure after the disengagement of the clutch, which causes fore-and-aft vibration and pitching of the vehicle body.